Mike Hookem, a UKIP MEP for Yorkshire and the Humber, said a constituent of his had been alerted to the Bristol area postal voting packs.

"Any ballot papers which have gone out with marks showing either remain or leave boxes being ticked in the example need to be recalled and the person responsible needs to be investigated" Mr Hookem said.

"It may be an isolated incident around Bristol but in the interests of democracy we cannot take that risk."

The Leave.EU campaign tweeted images of the postal ballot instructions and called on supporters to tell them if they had received the packs.

Analysis by Political Reporter Brian Wheeler

Both sides in this referendum campaign are on high alert for any hint of bias, particularly in official documents.

So it is, perhaps, no surprise that Bristol City Council's leaflet has caused such a row.

The council insists the graphic is entirely innocuous - but UKIP says it is a clear example of establishment bias in favour of the Remain campaign.

There is even a suggestion that it could leave the result of the referendum open to legal challenge.

The Electoral Commission has told Bristol council not to use it again. But with reports of similar leaflets dropping on doormats around the country it may be too late to do anything about it.

A spokeswoman for the Electoral Commission said: "While it is unlikely that postal voters in Bristol might be persuaded to vote a different way because of the graphic in these instructions, clearly it shouldn't have been used.

"We are following up with the counting officer for Bristol City to ensure that it is amended before any further postal voting packs are issued to voters in the area."

A Bristol City Council spokesperson said: "This form is designed to explain the logistics of voting by post and not to suggest how someone should vote.

"The placement of the pen graphic was entirely incidental and we do not believe that anybody could reasonably be influenced by such a graphic.

"However, given current sensitivities, for all future postal vote dispatches the form and graphic will be amended."